1. Field of the Invention
The present invention relates to an image forming apparatus and a method for controlling an image forming apparatus.
2. Description of the Related Art
Conventionally, there are image forming apparatus that read an original document with a sensor such as a CCD, generate image data, and form an image on a sheet by using the generated image data. There are also image forming apparatus that receive print data from a PC on a network, analyze and expand the received print data to generate image data, and form an image on a sheet by using the generated image data. In such image forming apparatus, the number of pages that can be read per unit time has increased because it has become possible to drive sensors such as CCDs to read original documents at high speed. In addition, because the speed of CPUs provided in image forming apparatus has increased and the cost of the faster CPUs has decreased, the print data received from a PC on a network can be expanded at high speed.
In light of these circumstances, there is a growing demand to increase productivity in operations to form images on sheets.
In order to improve productivity in an image forming operation, an image forming apparatus disclosed in Japanese Patent Laid-Open No. 2002-347987 is known that forms images at a shortened sheet conveyance interval (an interval between two sheets that are successively fed). By shortening the sheet conveyance interval, the number of image formed sheets per unit time increases, resulting in improved productivity of the image forming apparatus.
With the conventional image forming apparatus, however, when setting the sheet conveyance interval to be short, the extent to which the sheet conveyance interval can be shortened is limited.
For example, when developer is applied to a sheet and the developer is fixed, because heat is absorbed from the fixing unit during the fixing process of the developer, it is necessary to increase the temperature of the fixing unit to a temperature necessary to fix the developer until an image is formed on the next sheet.
For this reason, with the conventional image forming apparatus, the sheet conveyance interval can be set only to intervals during which the temperature of the fixing unit can be increased sufficiently.
In other words, the conventional image forming apparatus determines the sheet conveyance interval without taking into consideration a blank area that exists outside of the area in which an image is formed on a sheet.
If images formed on sheets include a blank area, the temperature of the fixing unit can be increased to a temperature necessary to fix the developer during the time between the formation of an image on a sheet and the formation of an image area on the next sheet. In this case, due to the length in the conveyance direction of the blank area, there is enough time to increase the temperature of the fixing unit.